Because of the barrier of the Himalayas. The region of India/Pakistan/Nepal/Bangladesh is called the "Indian subcontinent" because it's almost a world apart from the rest of Asia, separated by the biggest mountain range in the world. The mountains prevented people on either side from mingling to any great degree, and therefore resulted in the separate development of cultures and societies on both sides.
That's not to say there was no contact, of course it's possible to traverse the mountains. You can cross the Himalayas via Nepal to the Tibetan Plateau, which was one route taken by early Buddhist monks in spreading Buddhism from India to China. You can take the Khyber Pass across northwestern Pakistan into Afghanistan. You can travel from the east coast of India via ship to southeast Asia, and then on to China. And you can also cross the dense jungles past India's eastern states into Burma.
But all of these routes are difficult and generally discouraged large scale trade or the movements of armies until fairly recently.
In general, long distance travel overland was very difficult in the ancient world, even in the absence of formidable geographical barriers like the Himalayas. Sea routes, on the other hand, made it easy. There is a fairly easy sea route between the Indus Valley and Mesopotamia. From Indus Valley ports through the Persian Gulf to Mesopotamia it's a relatively short route of around 2,000 km, and you can hug the coast all the way never venturing very far from land. This is the route taken by ships during the early phases of the IVC. Later on it became even easier as ports were developed midway on the Arabian peninsula, in Bahrain and Muscat, and these places became important intermediaries in the IVC-Mesopotamia trade.
In contrast, the sea route from the IVC to the Yellow River Valley is around 12,000 km. The land route is blocked by the Tibetan Plateau or the Taklamakan Desert, and then there's still the Himalayas to cross. It's a very difficult route that could take years to traverse even for individuals, not to mention a whole trade caravan.
Another consideration is that the IVC is very old. The oldest Chinese civilization in the Yellow River Valley is traditionally considered to be the Xia dynasty from around 2,000 - 1,600 BC, though some historians doubt it existed. The Indus Valley Civilization is over a thousand years older, from around 3,500 BC. The Mature Harappan Phase extends from around 2,600 BC to 2,000 BC (though modern scholarship indicates it might be older), so it was coming to an end around the time the Yellow River civilization was starting. There is a mismatch between time periods.
That said, there is indeed evidence that some level of trade existed. Jade of Chinese origin has been found at IVC sites in the Swat Valley. Mineralogy indicates that the source of the jade was somewhere in the Yellow River region, from the neolithic Yangshao Culture. Possibly this arrived via the eastern route, that is, through the Indian state of Manipur bordering southeast Asia, since it's the oldest land route we know of through which India traded with the east. But the IVC is much older still, so who knows. At any rate, a few items don't exactly indicate a large, thriving trade such as existed between the IVC and Mesopotamia. It just shows some level of contact.
This is an interesting paper on the subject if you want to read more:
Another good source is the book Trade And Trade Routes In Ancient India by Moti Chandra.